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from the they're-dark-Jim,-but-not-as-we-know-it dept.
About 321 million light-years away from us is the Coma Cluster, a massive grouping of more than 1,000 galaxies. Some of its galaxies are a little unusual, however: they're incredibly dim. So dim, in fact, that they have earned the title of "Ultra-Dark Galaxies" (UDGs). (The term is actually "Ultra-Diffuse Galaxies", as their visible matter is thinly spread, though "ultra-dark" has been used by some sources and, let's face it, sounds a lot better). This was discovered earlier this year in a study that identified 47 such galaxies.
Dimness isn't necessarily unusual in a galaxy. Most of a galaxy's light comes from its stars, so the smaller a galaxy is (and thus the fewer stars it has), the dimmer it will be. We've found many dwarf galaxies that are significantly dimmer than their larger cousins.
What was so unusual about these 47 is that they're not small enough to account for their dimness. In fact, many of them are roughly the size of our own Milky Way (ranging in diameter from 1.5 to 4.6 kiloparsecs, compared with the Milky Way's roughly 3.6) but have only roughly one thousandth of the Milky Way's stars. The authors of the recent study interpret this to mean that these galaxies must be even more dominated by dark matter than are ordinary galaxies.
Original Submission
(Score: -1, Troll) by Anonymous Coward on Wednesday July 15 2015, @02:23AM
It's only dim matter, see, do you see? Moronic fuckwads and your dark matter obsession.
(Score: -1, Troll) by Anonymous Coward on Wednesday July 15 2015, @02:30AM
Authors of studies love funding because getting paid to write fraudulent studies saves them from having to any honest work.
(Score: 1, Insightful) by Anonymous Coward on Wednesday July 15 2015, @03:14AM
Where's the evidence supporting this hypothesis? Since its just so damn obvious, surely you won't have any trouble showing proof.
(Score: 2, Interesting) by Anonymous Coward on Wednesday July 15 2015, @11:27AM
Different AC. It is not my area of expertise, but I know the consensus model of cosmology is now built up on a huge number of ad hoc explanations:
http://arxiv.org/pdf/1204.2546v2.pdf [arxiv.org]
(Score: 5, Informative) by stormwyrm on Wednesday July 15 2015, @04:02AM
Sure it does. Scientists didn't hypothesise dark matter just to be cute, and in fact the lack of direct confirmation of its existence bothers them as it must. But the indirect evidence for dark matter is persuasive, and it seems to explain many features of the universe better than alternative hypotheses.
One big thing is this: the universe does not contain anywhere near enough visible matter for it to be expanding at the rate we observe. If visible matter was really all there is, then the universe should have expanded far quicker than we see. Now, you could say that this otherwise invisible matter is just "dim" ordinary matter, but that doesn't wash either. The present theory of the Big Bang uses well-established theories behind nuclear fusion reactions (yes, the same theories that are the basis for hydrogen bombs) to predict the abundances of various elements in the earliest stages of the universe, and attempting to fudge these parameters to make ordinary matter sufficiently common to account for the slower rate of expansion of the universe produces results that don't agree with what we observe. Also, there would have to be so much of this ordinary matter that we should have definitely seen it by now. Ordinary matter interacts with other ordinary matter you see, and if we really had as much of it as is required it wouldn't stay "dim" for very long. No, either there is dark matter, or gravity behaves weirdly at galactic scales. But even that latter hypothesis is strained.
There are objects out there in the universe such as the Bullet Cluster [wikipedia.org] where dark matter is the clearest explanation for what we can see. There is clear gravitational lensing present in observations of that cluster, but no visible matter around to account for the effect. Alternative theories of gravity like MOND have to strain to explain what is seen here, talking about extra fields that have additional energy and hence warp spacetime and produce the lensing in the Bullet Cluster and similar objects. But you know, there's a very useful term for extra fields whose energy warps spacetime: "dark matter".
The cosmic microwave background, the afterglow of the Big Bang as it were, is very nearly completely uniform, but it has very slight deviations (anisotropies) in it that are neatly explained by hypothesising dark matter. In the early universe, ordinary matter will experience pressure due to the influence of other forces under the pull of gravity, and hence will oscillate, but dark matter will just collapse under gravity because it isn't subject to these other forces. This results in subtle peaks in the CMB spectrum that are difficult to account for in any other way.
It is understandable why the dark matter hypothesis is unpalatable. People didn't like it when Copernicus proposed that the earth was not the centre of the universe, and it similarly bothers folks to consider that we aren't made up of the predominant type of matter in the universe either. It naturally bothers even its proponents that they cannot yet explain its true nature in spite of so much indirect evidence for its existence. However, the true test of any scientific theory is its agreement with observation and experiment, and thus far, the hypothesis of dark matter is holding up better than any alternatives. If you would like to propose an alternative hypothesis that doesn't posit the existence of dark matter, you are free to do so, but keep in mind that previous attempts at making such a theory such as MOND and TeVeS have to strain to explain all the observed phenomena that dark matter is able to explain.
Numquam ponenda est pluralitas sine necessitate.
(Score: 1, Interesting) by Anonymous Coward on Wednesday July 15 2015, @11:58AM
According to figure 1 of Angus and Diaferio (2011) MOND + sterile neutrinos can fit this data as well:
http://mnras.oxfordjournals.org/content/417/2/941.full [oxfordjournals.org]
(Score: 0) by Anonymous Coward on Wednesday July 15 2015, @04:12PM
(Score: 1, Interesting) by Anonymous Coward on Wednesday July 15 2015, @04:47PM
So you don't need dark matter because you can describe the observations with a theory including dark matter in the form of sterile neutrinos?
So now the competing theories are "MOND and dark matter" vs. "just dark matter"? Well, Occam's razor tells me quite clearly which of those two options I should prefer.
(Score: 0) by Anonymous Coward on Wednesday July 15 2015, @05:27PM
It is not *cold* though.